Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for components used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed.
Ceramic matrix composites (CMCs) are a class of materials that consist of a reinforcing material surrounded by a ceramic matrix phase. Such materials, along with certain monolithic ceramics (i.e. ceramic materials without a reinforcing material), are currently being used for higher temperature applications. Some examples of common CMC matrix materials can include silicon carbide, silicon nitride, alumina, silica, mullite, alumina-silica, alumina-mullite, and alumina-silica-boron oxide. Some examples of common CMC reinforcing materials can include, but should not be limited to, silicon carbide, silicon nitride, alumina, silica, mullite, alumina-silica, alumina-mullite, and alumina-silica-boron oxide. Some examples of monolithic ceramics may include silicon carbide, silicon nitride, silicon aluminum oxynitride (SiAlON), and alumina. Using these ceramic materials can decrease the weight, yet maintain the strength and durability, of turbine components. Therefore, such materials are currently being considered for many gas turbine components used in higher temperature sections of gas turbine engines, such as airfoils (e.g. compressors, turbines, and vanes), combustors, shrouds and other like components that would benefit from the lighter-weight these materials can offer.
CMC and monolithic ceramic components can be coated with environmental barrier coatings (EBCs) and/or thermal barrier coatings (TBCs) to protect them from the harsh environment of high temperature engine sections. EBCs can provide a dense, hermetic seal against the corrosive gases in the hot combustion environment while TBCs can set up a thermal gradient between the coating surface and the backside of the component, which is actively cooled. In this way, the surface temperature of the component can be reduced below the surface temperature of the TBC. In some instances, a TBC may also be deposited on top of an EBC in order to reduce the surface temperature of the EBC to below the surface temperature of the TBC. This approach lowers the operating temperature at which the EBC must perform and as a result, can increase the operating life of the EBC.
Currently, most EBCs consist of a three-layer coating system including a silicon bond coat layer, at least one transition layer comprising mullite, barium strontium aluminosilicate (BSAS), a rare earth disilicate, or a combination thereof, and an outer layer comprising BSAS, a rare earth monosilicate, or a combination thereof. The rare earth elements in the mono- and disilicate coating layers may comprise yttrium, leutecium, ytterbium, or some combination thereof. Together, these layers can provide environmental protection for the component.
TBCs generally consist of refractory oxide materials that are deposited with special microstructures to mitigate thermal or mechanical stresses due to thermal expansion mismatch or contact with other components in the engine environment. These microstructures may include dense coating layers with vertical cracks or grains, porous microstructures, and combinations thereof. The refractory oxide material typically comprises yttria-doped zirconia, yttria-doped hafnia, but may also include zirconia or hafnia doped with calcia, baria, magnesia, strontia, ceria, ytterbia, leuticium oxide, gadolinium oxide, neodymium oxide, and any combination of the same. Other examples of acceptable refractory oxides for use as a TBC can include, but should not be limited to, yttrium disilicate, ytterbium disilicate, lutetium disilicate, yttrium monosilicate, ytterbium monosilicate, lutetium monosilicate, zircon, hafnon, BSAS, mullite, magnesium aluminate spinel, and rare earth aluminates.
Regardless of composition or substrate, most EBCs and/or TBCs are generally applied using one of conventional air-plasma spraying (APS), slurry dipping, chemical vapor deposition (CVD), or electron beam physical vapor deposition (EBPVD). Unfortunately, none of these methods are without issue. For example, air-plasma spraying is generally limited to line-of-site applications. As most high temperature gas turbine engine components would benefit from both exterior and interior coating with a barrier coating, APS may not be the method of choice for such applications. Additionally, while slurry dipping can provide some cost savings and can cover additional areas of the component (i.e. internal passages) when compared to APS, it is designed for thin coatings. Since some high temperature gas turbine engine components would benefit from thicker coatings, slurry dipping may not be suitable for all applications. EBPVD and CVD tend to be more costly than APS and slurry dipping, and are generally useful for thin coating applications only due to slow deposition rates.
Furthermore, repairing EBCs and TBCs applied using traditional methods can be complex and costly, typically requiring the entire coating to be stripped and replaced.
Therefore, there remains a need for improved methods for repairing barrier coatings.